There is growing interest in signal generation in the millimeter-wave and terahertz frequency ranges. There are numerous applications for mm-wave frequencies such as broadband wireless access (e.g., WiMax), vehicular radars, short-range communications, and ultra-narrow pulse generation for ultra-wideband (UWB) radar. Bio- and molecular spectroscopy were the first and the main applications for the terahertz band, which is usually defined to be between 300 GHz and 3 THz. Recently, this range has also been used for imaging, compact range radars, and remote sensing.
Signal generation at these frequencies is a major challenge in solid-state electronics due to the limited efficiency and breakdown voltage of active devices, as well as the lower quality factor of passive components caused by ohmic and substrate loss. Currently, because of the advances in solid-state technology, it is possible to generate hundreds of milliwatts of power at around 100 GHz and below using GaAs and InP heterojunction bipolar transistor (HBT) power amplifiers and high electron mobility transistor (HEMT) power amplifiers. Fundamental oscillation using Gunn diodes, impact ionization avalanche transit-time (IMPATT) diodes, and tunneling transit-lime (TUNNETT) diodes, and more recently, silicon-based power amplification, result in high output power for the same frequency band (around 100 GHz). However, above 150 GHz, frequency multipliers presently surpass all other solid-state sources in terms of output power, and quantum cascade lasers with cryogenic cooling dominate for frequencies above 2 THz. Consequently, frequency multipliers are used to cover signal generation for a major part of the terahertz band.
Schottky barrier diodes are the main components of the terahertz frequency multipliers. Schottky barrier diodes are also used in terahertz sub-harmonic up/down converters. In addition, scaling of the CMOS process has resulted in higher transistor cut-off frequency, resulting in the recent inclusion of CMOS transistors in terahertz multipliers in the form of harmonic oscillators. Although frequency multipliers have the highest output power compared with other solid-state components in the terahertz band, frequency multipliers tend to generate lower power relative to other signal sources that are used in higher (>3 THz) and lower (<150 GHz) frequency regions. This is referred to as the “terahertz gap,” and is a result of a fundamental trade-off between series resistance and capacitance of the nonlinear device.
One approach to achieving higher power levels for the frequency multipliers is to employ multiple devices and then combine their output powers. Unfortunately, matching considerations, as well as the effect of propagation delay, limit the number of devices that can be combined using existing power-combining techniques. To address this issue, spatial power-combining techniques such as quasi-optical arrays have been used to achieve high powers in terahertz frequencies. However, quasi-optical arrays require antenna arrays, which makes this technique bulky and cost inefficient. Furthermore, generating a guided terahertz signal using this technique is challenging.
Accordingly, there continues to be a need for high frequency signal generation circuits in the millimeter and terahertz frequency bands capable of achieving high powers. In addition, cost and size considerations continue to be factors in high frequency generation.